1. Field of the Invention
This invention relates to a method of preparing overbased magnesium phenates. More particularly, it relates to a process for preparing overbased magnesium phenates wherein magnesium oxide is used as the source of magnesium.
2. Description of the Prior Art
The operation of diesel and spark ignition internal combustion engines is typically accompanied by the formation of sludge, lacquer and resinous deposits which adhere to the moving engine parts and thereby reduce engine efficiency. In order to prevent or reduce the formation of these deposits, a wide variety of chemical additives have been developed for incorporation into lubricating oils. These additives, which are commonly referred to as detergents or dispersants, have the ability to keep deposit forming materials suspended in the oil so that the engine remains in a clean and efficient operating condition for extended periods of time. Among the many additives which have been developed for this purpose, the alkaline earth metal phenates and particularly their sulfurized derivatives have been found to be highly effective detergents for lubricating oils.
In addition to serving as highly efficient detergent additives for lubricating oils, alkaline earth metal phenates are also excellent oxidation and corrosion inhibitors. Further, these phenates have the ability to neutralize acidic combustion products which are formed during engine operation. The formation of these acidic products is a particular problem during engine operation with high sulfur fuels. These acids appear to cause degradation of the lubricating oil and are corrosive to metal engine components such as bearings. If uncontrolled, the corrosion induced by acidic combustion products can cause rapid engine wear and a resulting early engine breakdown.
To further improve the ability of alkaline earth metal phenate additives to neutralize acidic combustion products, these additives are commonly overbased.
The term "overbased" is used to describe phenates containing an amount of alkaline earth metal which is in excess of that required to react with the phenol from which the phenate is derived. In addition, this excess alkaline earth metal is present in a form which is capable of neutralizing acids. Typically, the excess metal is in the form of its carbonate, and the overbased phenate comprises a colloidal dispersion of the metal carbonate in the metal phenate as a dispersant.
Overbased calcium and barium sulfurized phenates have been widely used as additives for lubricating oil. Indeed, prior to about 1967, substantially all of the phenates used commercially in crankcase oils were overbased barium or calcium phenates. Although these overbased calcium and barium phenates neutralize acidic combustion products satisfactorily and are effective detergents, they do not provide a sufficient degree of rust inhibition for satisfactory protection of the engine parts. In contrast, the corresponding overbased magnesium phenates do provide the desired degree of rust inhibition. In addition, the overbased magnesium phenates are preferable over their calcium and barium counterparts because the magnesium containing compositions, as a consequence of the lower atomic weight of magnesium, afford a smaller quantity of inorganic ash for a given capacity to neutralize acid. Unfortunately, these magnesium phenates have been more expensive and far more difficult to prepare than their calcium and barium counterparts.
Many of the prior art processes for the preparation of overbased magnesium phenates involve the use of a magnesium alkoxide as a source of magnesium. Representative examples of this approach are set forth in U.S. Pat. Nos. 2,916,454 (Bradley et al.); 3,718,589 (Rogers et al.); 3,746,698 (Hunt et al.); 3,932,289 (King et al.); 4,104,180 (Burnop) and 4,196,089 (Pitzer et al.) and in British patent specification No. 2,055,886. However, such processes are unsatisfactory because they typically involve the use of magnesium metal, an expensive starting material, for the preparation of the magnesium alkoxides.
In view of its availability and low cost, magnesium oxide represents one of the most desirable sources of magnesium for use in the preparation of overbased magnesium phenates. However, as stated in the above mentioned U.S. Pat. No. 4,196,089 to Pitzer et al., efforts to produce overbased magnesium phenates having a total base number (TBN) in the range of about 200 to about 275 by reacting sulfurized oil-soluble aliphatic hydrocarbyl-substituted phenols with magnesium oxide combined with carbonation with carbon dioxide, even at quite high temperatures, have been unsuccessful.
The amount of alkaline material present in compositions such as overbased magnesium phenates is conventionally expressed in terms of a total base number (TBN). This is defined as the number of milligrams of potassium hydroxide which are equivalent to the amount of acid required to neutralize the alkaline material present in one gram of the composition. Consequently, the magnitude of the total base number serves to indicate the ability of a given composition to neutralize acids. A standard procedure for measuring TBN is set forth in American Society for Testing and Materials (ASTM) test D-2896.
U.S. Pat. No. 3,388,063 to Allphin discloses the preparation of highly overbased magnesium alkylphenates by a process which involves combining magnesium oxide, a dihydric alcohol, a relatively high molecular weight monohydric alcohol and a small amount of an alkaline earth metal sulfonate in a hydrocarbon medium, heating the mixture to drive off water and a major portion of the dihydric alcohol, adding a sulfurized alkylphenol at an elevated temperature, carbonating the composition with carbon dioxide and, finally, removing volatile materials. This process requires the use of a monohydric alcohol containing from 8 to 18 carbon atoms and a dihydric alcohol of from 2 to 3 carbon atoms. In addition, the process of this patent does not utilize water as a reactant.
U.S. Pat. No. 4,049,560 to Dominey discloses a process for the preparation of overbased magnesium phenates which involves the reaction of carbon dioxide with a mixture which comprises: (1) a sulfur-containing phenol such as a sulfurized phenol which contains one or more hydrocarbyl substituents; (2) a sulfonic acid, sulfonate or sulfate; (3) an alkanol such as methanol; (4) magnesium oxide or hydroxide; (5) a carboxylic acid, anhydride or salt; and (6) a diluent oil. However, the process of this patent does not involve removal of the alkanol prior to carbonation, does not utilize water as a reactant, but does require the use of a carboxylic acid, anhydride or salt as a promoter. Further, the products of this process have a relatively low total base number of about 200 to 250.
U.S. Pat. No. 4,137,186 to Sabol discloses a process for preparing overbased magnesium sulfonates which first involves forming a mixture which contains an oil-soluble ammonium sulfonate, a magnesium compound such as magnesium oxide, a lower alkanol such as methanol, and an inert diluent. This mixture is heated to hydrate the magnesium oxide, after which the lower alkanol is removed. Finally, the process is completed by addition of an acidic material such as carbon dioxide at a temperature between about 80.degree. and 155.degree. F. (27.degree.-68.degree. C.). This patent, however, contains no mention of a phenol, a sulfurized hydrocarbyl-substituted phenol or metal salt thereof and fails to suggest that a similar process could be utilized to prepare overbased magnesium phenates. U.S. Pat. No. 4,201,682 to Sabol et al. discloses a similar process for preparing overbased magnesium sulfonates but does not disclose the removal of the alkanol prior to carbonation with carbon dioxide.